Science Behind Online Slots: Explaining Random Number Generators

how does a random number generator work slots

Added: May 25, 2022

Last updated: April 19, 2023

The Random Number Generator is the heart of every slot: every result from every spin is based on its outcome

All types of slots are powered by the Random Number Generator: land-based slots (such as in Las Vegas) and video slots from one of the larger providers such as Netent, Microgaming or Quickspin.

The Random Number Generator is also called a “random generator” and sometimes abbreviated to “RNG.”

In addition to slots, it is the crucial element of electronic versions of table games such as Roulette, Blackjack, Keno and scratch cards.

The Random Number Generator is even used in some live dealer casino games, such as Lightning Roulette and Lightning Baccarat, to determine the number and multiplier size of the  Lightning Strikes.

What does the Random Number Generator do?

The Random Number Generator ensures that each spin on the slot links to a random number. The random number (a unique outcome of an algorithm) then translates into a result of the spin (or the bet). It could be a prize or no prize (a loss of the bet). The outcome of the spin then translates into a visual display on the screen.

Finally, the outcome of the spin (or the bet) is settled with your slot balance.

This outcome is determined purely by allowing the RNG to choose a number from billions of unique numbers. Each unique number translates into an outcome.

An RNG has no memory and cannot look into the future. Each number is drawn independently.

As a result, there are no “hot” or “cold” slots, and it is impossible to know when a slot is going to pay. It all comes down to playing a slot at the right place and the right time.

How do you visualize the Random Number Generator?

Imagine throwing a 10-sided dice 14 times in a row. The dice have as values a “0” up to “9”. You throw it 14 times, and the 14 numbers you get are the numbers after the digit.

If you have Excel, and you write: RAND() in one of the empty cells of your spreadsheet, you get a random number between 0 and 0.99999999999999.

Mathematicians, most of the time, refer to Monte Carlo simulation. The RNG simulates numbers between 0 and 0.99999999999999. Each number is independent. RNG’s are also used in many other fields (finance, credit, marketing).

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Example of how the Random Number Generator works for slots

Explanation of the case

We have made a simplified example of how a slot makes use of the RNG. The RNG always draws a number between 0 and 0.99999999999999 (14 numbers after the digit). As explained above, this number is purely random.

Imagine a slot where you have the following 10 outcomes: 0 (a loss and no prize) and 9 prizes of either 0.2x, 1x, 2x, 5x, 10x, 20x, 100x, 200x and 500x the bet.

You see the probability of you getting these prizes in the table below.

how does a random number generator work

For example, the chance of getting a prize of 20x the bet is 0.3% (see the light blue column), which is once every 333 spins (see the grey column).  In this slot, if the RNG draws any number between 0.995000000000001 and 0.9980 (see yellow column), you get a prize of 20x the bet (green column).

The theoretical RTP is 97.8% in this slot (calculated in the orange column).

This is all to it when you play a slot. The RNG has determined your prize as soon as you pushed the spin button.

How do the prizes translate into visuals?

A slot would be dull and boring if there would be no more to it. As you know, slots have their graphics and sounds and features etc.

Let’s follow up with this example further, where we show how the result the RNG generates is translated to a visual display.

Imagine a straightforward slot with three reels with Bars, Sevens, Strawberry’s, Cherries and maybe some other fruit symbols. There is one payline only.

Below, the combinations you get are attached to the prize you win.

how does a rng work for slots

two reels, therefore, stop (automatically) at the Sevens. The third reel stops anywhere else (also randomly at anything but a Seven).

Example with 10 spins with this simplified slot

Below, you see the result of 10 spins with this slot.

We have drawn 10 random numbers. We were lucky in this example: in the 9th spin, the random generator has the value: 0.998672599665502. This implies a prize of 100x the bet. It translates into 2 bars on the first two reels.

It is needless to say that in a real slot machine, there are many more possible outcomes (thousands and thousands, dependent on the complexity of the slot). Besides that, you deal with features, free spins, multipliers, wilds and much more.

The principle is the same, though: the Random Number Generator randomly stops at a number, which translates into a fixed outcome.

This outcome is then again translated into symbols, features, free spins, multipliers, wilds, etc. What happens after that is purely for show and entertainment. Smoke and mirrors, if you will.

Some slots give you the feeling that you can determine the outcome (with pick and click bonuses, for instance). Time for that myth to sleep with the fishes: your choices have zero impact on the result.

The RNG cannot be influenced and cannot be rigged

It is essential to know that a Random Number Generator is always independent and cannot be changed. Not by the player, but even more important, not by the casino.

The software that uses the RNG runs on external servers of the game providers (such as Netent, Microgaming, Quickspin en Play’n GO).

As a result, the online casino does not have access to the games. Online casino’s only acts as a financial intermediary. They take care of the registrations, the storage of personal data, and the transactions’ financial handling.

The importance of a properly functioning Random Number Generator in the gambling world

Without a properly working Random Number Generator, the outcome of the algorithms could be predicted.

In that case, players would be able to predict the outcome of spins and could make the casino bankrupt in no time.

After all, if you know what the RNG outcome will be, you can adjust stakes accordingly. A well-functioning RNG, therefore, ensures that you can never know in advance what a slot will payout. 

The Random Number Generator is essential for the fairness of any automated casino game.

What role does the RNG play in the house advantage?

A slot is programmed in such a way that there are more losing combinations than winning combinations. The difference that remains at the bottom of the line is the theoretical House Edge. In the example, you saw that the theoretical RTP was 97.8%, and the House Edge, therefore, 2.2%.

This built-in house edge (for online slots, usually around 3% – 4%) ensures that online casinos will always make money on a slot in the longer run.

The random and independent nature of the RNG makes it possible for players to beat the casino. After all, it’s entirely possible to be on the right side of the RNG in the short term and get a relatively large number of winning combinations.

In fact, in theory, it is possible to win the highest jackpot twice in a row. After all, the RNG starts a new cycle every turn and has no memory. The RNG does not know if you have just won or lost. It can’t make his choices accordingly.

Stopping on time at winnings is the most crucial success factor. The longer you play, the sooner the law of large numbers will apply. You will inevitably run into the House Edge. Put simply, if you do trillions of spins, you will ALWAYS lose.

How do you know if the slot is truly random and not rigged?

To check whether slots and video slots are random, they are supervised by various supervisors (agencies). The Nevada Gaming Commission tests slots in Las Vegas casinos on a random basis. The UK Gambling Commission does so for online casinos who have a license to operate in the UK.

Look for casino’s with a license of the Malta Gaming Authority (MGA) or the UK Gambling Commission

You can be confident that you are not rigged in online casinos if the casino is supervised by one of the stricter regulators. The more stringent the supervisor, the more demanding the software will be. Therefore, we advise you to play at reliable online casinos that have a Malteser license or a license in the United Kingdom.

Avoid self-produced software of online casino’s and play games from respected gaming providers

Besides, we advise you ALWAYS to avoid those online casinos that use self-produced software. It is safer to play at an online casino that offers games from respected gaming providers such as Netent, Quickspin, or Play’n GO.


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